Is it the tightening of the head bolts that stretches them or is it the running of the engine and the detonation 'pushing' the head from the block? Or both?

In other words: If you have stretch bolts (as with my Golf) and it's barely run (20mins max) will they have stretched yet? So will I need to replace them if I take the head off?

Dumb question probably!

Thanks!
James

I would change them to be on the safe side. a new set is cheaper than a head gasket. why do you want to take the head off again? didn't silicone work on the water outlet? I think it's the tightening of the bolts that stretch them .

tightening streches them a fair bit,they can snap if you use them twice

Entirely depends on what VW say in the manual.

A lot of urban myths around about stretch bolts, a stretch bolts aren't stretched into the plastic zone that much that they start to "neck" and weaken but each stretching changes the mechanical properties of the material actually
increasing the elastic limit i.e.the load point where elastic deformation gives way (no pun intended) to plastic deformation.

Each time a bolt is stretched from the elastic to plastic zone it will ratchet up the elastic limit slightly and this is the problem do it a few times and the bolt will no longer plastically deform during tightening, thus defeating the purpose of using stretch bolts.

With the stretch bolts on a Rover K the bolts can be used a few times provided they pass the length tests detailed in the Rover manual in addition the threads should be cleaned with a proper hex die-nut.

[Edited on 26/10/05 by britishtrident]

Alot of stretch bolts are allowed to stretch to a certain length as Trident says.

If theyr not to pricey id use new ones.

Heh heh lots of bill bull5hit around about bolts!

Britishtrident is quite right, here's my complementary take!

All bolts stretch, that's how they stay tight and hold things together.

"Normal" torque settings are derived through very complex relationships (all given in the Bosch Automotive "Bible" if you have one, and are based on stretching the bolt to a given % of it's elastic limit, pretty close to the maximum elastic strain but allowing for error as lubrication and surface finish can give errors of over 300% in torque value!!!

"Stretch Bolts" are actually "Torque to Yield" bolts, which obviously stretch as all bolts do, but are easier to tighten correctly as they are designed to give the correct clamping force at their yield point, i.e. the point on the stress-strain curve where the strain increases for no increase in stress. At this point the curve flattens off and the clamping force remains relatively constant over a comparatively (but not really that big) large strain, or "amount of tightening turn".

This means that it is easier to get clamping pressures even over several head bolts for example, which may have slightly different oil residues on them which could lead to wildly inaccurate clamping forces if torqued up "normally".

The downside is that once you take a piece of steel (i.e. a bolt) up to this strain, it is stretched beyond its elastic limit and it will not return to normal. It's properties have changed so the previously applied tightening procedure will not give the right clamping force if applied to already yielded bolts.

[Edited on 26/10/05 by NS Dev]

"TORQUE TO YIELD BOLTS"
A name that that engineers chose as it actually describes the part and how it’s used. The yield strength or offset yield strength is the stress required to produce a small-specified amount of plastic deformation. A good way of looking at offset yield strength is that after a specimen has been loaded to its 0.2 percent offset yield strength and then unloaded it will be 0.2 percent longer than before the test. A torque to yield bolt is physically longer after use then before.
So will it break? Well since the material these bolts are made from isn’t published your guess is as good as mine, but if an engine manufacturer says not to re-use them why would you? Since they also consider warranty and material cost, I highly doubt they would pay for it under warranty if it weren’t required. I know Ford North America insists this type of bolt is replaced during engine repairs. Its clearly stated in the repair manuals, often with a warning.
Why use these bolts on an engine? Well they hold their torque better after heat cycling, don't require retorquing, have a higher clamping load compared to the same bolt torqued with less force, and fewer are required to attain the same load.



[Edited on 27/10/05 by madman280]